Body condition reveals hidden correlations between co-infection and behavior in sunfish

The role of parasites in maintaining consistent inter-individual differences in behavior (i.e., personality) is the subject of increasing study and debate. While behavioral differences may expose individuals differently to parasites, parasite infection can itself change host behavior, sometimes favoring the parasite’s own transmission. Furthermore, parasites can alter the functioning of energetically costly organs like the brain, thus affecting cognitive performance. However, relationships among infection, cognition, and behavior can be complex and difficult to interpret, especially in wild populations where individual health status is unknown. The inclusion of body condition as a fitness proxy may help reveal relationships between parasites and host traits that are otherwise masked. We examined relationships among host body condition, personality (i.e., exploration, boldness), cognition (i.e., aversive learning), and parasite density in wild-caught pumpkinseed sunfish (Lepomis gibbosus) naturally infected with endoparasites. We measured exploration (i.e,. surface explored in cm), boldness (i.e,. latency to emerge from shelter in seconds) and cognition (i.e. aversive learning; difference between trial 1 and 2 in latency time to enter the black side). Exploration and boldness were measured twice (trial 1 and 2). Cognitive performance had one learning trial, and two test trials (24-hour delay between trials). Exploration in an open field test was repeatable in sunfish. Boldness, assessed using a shelter test, was not repeatable, but was correlated with exploration. Host exploration decreased with both increasing parasite density and decreasing body condition. Only individuals in relatively lower body condition displayed a negative relationship between parasite density and exploration, suggesting a pathologic effect of the parasites on the sunfish. Aversive learning was not influenced by co-infection. Our results show that body condition is important to consider when studying wild populations as some patterns observed between parasite density and host behavior were only revealed when body condition was taken into consideration.